The invention relates to the infiltration compositions used both when making composite material parts having a matrix that is ceramic or at least partially ceramic, referred to below as ceramic matrix composite (CMC) material and when making parts made of carbon/carbon (C/C) composite material.
The field of application of the invention is making parts that are to be exposed in service to high temperatures, in particular in the fields of aviation and space, in particular hot parts for aeroengines, it being understood that the invention may be applied to other fields, e.g. the field of industrial gas turbines.
CMC and C/C composite materials present good thermostructural properties, i.e. strong mechanical properties that make them suitable for constituting structural parts, together with the ability to conserve those properties at high temperatures.
Using CMC or C/C materials instead of metals for making parts that are exposed in service to high temperatures has therefore already been recommended, particularly since CMC and C/C materials present significantly lower density than the metal materials they replace.
A well-known method of fabricating CMC material parts consists in making a preform from fiber plies of carbon fibers or silicon carbide (SiC) fibers and in infiltrating the preform with a composition based on molten silicon so as to form a ceramic matrix. Such a densification process is known as melt infiltration (MI). By way of example, reference may be made to the following documents: U.S. Pat. No. 4,889,686, U.S. Pat. No. 4,944,904, or U.S. Pat. No. 5,015,540. The infiltration composition is based mainly on silicon, since that element presents a coefficient of thermal expansion that is close to that of the fibers of the preform. The infiltration composition may correspond to silicon on its own or to a silicon alloy that generally contains a small amount of one or more other elements such as titanium, molybdenum, boron, iron, niobium, etc.
Densification by the MI process presents the advantage of being faster and easier to perform than densification by chemical vapor infiltration (CVI). Nevertheless, the silicon-based infiltration composition that is used can present a melting point or temperature that is higher than the thermal stability temperature of the fibers of the preforms (heat stability). Under such circumstances, the fibers may be subjected to degradation while the molten composition is being infiltrated into the preform, which can reduce the mechanical properties of the fibers by a considerable amount.